The present invention is an improvement over the prior art Josephson junction memory cells. A key component of the prior art Josephson junction memory cell is a superconducting loop containing one or more Josephson junction(s) that stores a persistent circulation current. If the inductance of this loop is sufficiently small, the associated trapped flux assumes a discrete set of values proportional to the single flux quantum (SFQ) and can be used to encode digital information. The logic state of the cell is changed by switching the Josephson junction(s) from its(their) superconducting to resistive state(s); this process in a superconducting quantum interference device (SQUID) occurs at subnanosecond time-scales. In addition, very little energy (&lt;10.sup.-18 joules) is dissipated during this switching event. Nonetheless, reliable operation is anticipated due to the low level of thermal noise at the cryogenic operating temperatures. Moreover, there is no energy consumption associated with the information storage and the memory is non-volatile. Therefore such SFQ circuitry offers promise for ultrafast memory with high densities that are not limited by the conventional problem of power-dissipation and resultant heat removal.
Though the possibility of SFQ data storage was first proposed by D. J. Dumin et al. in an article entitled "Application of Quantized Trapped Flux in a Superconducting Memory" in J. Appl. Phys., 34, 1566-67 (1963) shortly after the discovery of the Josephson effect in an article by B. D. Josephson entitled "Relation Between the Superfluid Density and Order Parameter for Superfluid H.sub.e Near T.sub.c ", in Phys. Lett. 21, 6, 608-609, (1966), the main obstacle hindering the practical realization of such devices has been the inconvenience of information read-out. The first SFQ memory cell proposed was the Flux Shuttle, by T. A. Fulton et al. in an article entitled "The Flux Shuttle-A Josephson Junction Shift Register Employing Single Flux Quanta" in Proc. IEEE, 61, 28-35 (1973) and in an article by T. A. Fulton et al. entitled "Experimental Flux Shuttle" in Appl. Phys. Lett, 22, 232 (1973), a transmission line comprising of a parallel connection of the superconducting loops. In effect, the cell performed as a shift register where the position of the SFQ could be controlled by current-biasing a specific junction in the circuit and then driving the junction resistive with a current-pulse. Though several other static SFQ memory cells have been proposed subsequently, they all share a common feature: though digital bits are stored as SFQ, the information is read out as a DC voltage level via a Josephson gate. In order to perform its function as a transducer, the sensing junction must be in the hysteretic regime; thus its reset can only be achieved by switching off the bias current, a process that severely limits the clock frequency of the memory cell.
Josephson Junction devices must use single-flux quanta (SFQ) for both information storage and retrieval in order to maintain their advantage in speed compared with conventional semiconducting transistor memories. Prior art dynamic SFQ circuitry offers such a possibility, where information is passed between logic elements in the form of short voltage pulses with fixed area equal to a single flux quantum (SFQ) as described in an article by K. K. Likharev and V. K. Semenov entitled "RSFQ Logic/Memory Family: A New Josephson Junction Technology for Sub-Terahertz-Clock Frequency Digital Systems" in IEEE Trans. on App. Supercond., vol. 1, no. 1, pp 3-28, March 1991. Recently a dynamic SFQ memory cell has been proposed by S. V. Polovsky et al. In an article entitled "Rapid Single Flux Quantum Random Access Memory" in IEEE Trans. on App. Supercond., 5, 3000-3005 (1995) wherein SFQ pulses code, store and retrieve binary information in a network of single-junction SQUID loops. Though this design represents an important step in the practical realization of high-speed SFQ data storage, its read-out mechanism is fault-intolerant and thus unreliable due to ever present pragmatic difficulties with trapped flux in such an array. Furthermore its read-out procedure is destructive which limits its possible applications to high-density main memory where speed is not a premium concern.